The present invention relates to an optical probe adapted for use in an enclosed space, for example, a cavity to allow measurement of features such as flow rate, particle size and concentration of substances contained in the space. It is especially, but not exclusively, concerned with PIV. It is concerned with planar light sheet anemometers (PLSA), especially with miniature PLSA""s.
Conventional cavity inspection devices (or devices for use in confined spaces) have allowed images to be taken/the cavity to be visualised and include endoscopes having a number of prisms and mirrors which conduct white light from a source into a cavity. The prior art endoscopes allow the user to see what is in the cavity with the naked eye or can be used in combination with a camera allowing visualisation of the cavity on a screen or a photograph.
Conventional fluid flow analysis systems such as Laser Doppler Anemometers (LDA), Particle Image Velocimeters (PIV) and Phase Doppler Anemometers (PDA) are large and when it is desired to obtain flow information in a confined space are incapable of obtaining a wide enough range of information sought, and are practically impossible to use due to the lack of optical access available in enclosed cavities (e.g. in the bearing chamber of an aero-engine).
It would therefore be beneficial in a wide variety of fields, including engineering and medicine, to have a probe system which may or may not allow the visualisation of the inside of an enclosed cavity, but more importantly allows measurements to be taken relating to the contents of the cavity, for example for a fluid-containing cavity, the particle velocity, the particle size, and particle concentration, of the fluid in the cavity.
The present invention originated from work in the field of Particle Imaging Velocimetry (PIV). Particle Image Velocimetry (PIV) benefited from the development of LDV (Laser Doppler Velocimetry) and constitutes an answer to the need for Whole Field measurements. It was developed in the late 1970""s, was practically implemented in the early 1980""s, and its use started to spread in the late 1980""s. It is now a developed technique. The advantages of this type of measurement system are found in many domains: when using intermittent facilities flowfields may be measured without assuming perfect repeatability of testing conditions; in many instances testing times are much shorter than with other methods of flow/fluid measurement; and these techniques allow the access to quantities that were otherwise impossible to determine such as instantaneous vorticity fields. The technique typically images a particle at two different times and establishes the velocity of the particle by evaluating the images to establish how far the particle has travelled in a known time.
Particle Image Velocimetry and Laser Induced Fluorescence (LIF) are based, like Laser Doppler Velocimetry, on the measurement of the velocity of tracer particles carried by the fluid. However, rather than concentrating light in a small probe volume (as in LDV), a complete plane of the flow under investigation is illuminated in PIV and LIF. This is performed by creating a narrow light sheet which is spread over the region of interest, the sheet illuminating an isolated 2-D plane of interest. Tracer particles are therefore made visible and images of the illuminated particles are recorded. These recordings will typically either contain successive images of single tracers in time or successive frames of instantaneous images of the whole flowfield. The displacement of the tracer will then be determined through the analysis of these records.
PIV systems are known for providing information on a fluid in a confined space which have a first probe which comprises an emitter optically coupled to a laser and designed to emit a sheet of laser light, and a second probe, spaced apart from the first probe, and comprising a detector/receiver designed to detect scattered laser light and provide signals to a computer. The spacing between the emitter and the detector needs to be accurately controlled, as does their relative angular orientation and relative position.
Laser-induced fluorescence (LIF) imaging is another imaging technique which uses a sheet of light. It relies on the quantum nature of molecules and atoms, whereby energy transitions can only occur between certain quantized energy states. A diatomic molecule can have several modes of quantum energy. The three relevant to LIF Studies are electronic, vibrational and rotational. The first mode, the electronic state, is usually denoted by letter, with X being the lowest (ground) electronic state, A being the first excited state, B the second, etc. The molecule also has vibrational energy, denoted by the vibrational quantum number v, having integer values starting with 0. The third energy state is the rotational energy, denoted by the rotational quantum number J. Only certain energy transitions are allowed by the selection rules of quantum physics. A molecule in a low energy state can only be optically pumped up to a higher energy state by interaction with a photon of energy exactly equal to the energy difference between allowed energy states of the molecule, and an excited molecule can only relax by giving up a quantum of energy equal to the difference between allowed energy states, either by emission of a photon, or by collision with a neighbouring molecule.
Laser-induced fluorescence takes advantage of this phenomenon by optically exciting a species with photons of a frequency matching an allowable level difference of the species being probed. It should be noted that different species tend to have different energy transitions, so it is generally possible to chose a transition for a given species that is well isolated from possible transitions of other species that may be present. The resulting fluorescence caused as excited molecules relax by photon emission can be collected and analysed to determine local species concentration and/or temperature.
Laser-induced fluorescence utilises a sheet of laser light generated by a tunable laser source to illuminate a two dimensional plane through the sample, and uses a sensitive intensified CCD camera arranged at 90xc2x0 to the sheet of light to image the resulting fluorescence from the illuminated area. The processing of the acquired images is similar to PIV except for the additional filters and detectors for phase separation. There are several variations of the principle; namely LIF, PLIF (Planar Laser Induced Fluorescence), MLIF (Mixing Measurements using Laser Induced Fluorescence) etc.
In some PIV/LIF measurements it is necessary to move the sheet of light, and also move the detector in a corresponding manner so as to ensure optimum detection for the new position of the sheet of light. Careful alignment of the emitter and detector at their second (and subsequent) positions is also important, but critical to the measurement process.
When, for example, measuring lubricant (oil) parameters in a working engine (e.g. a working test-bed aeroplane engine) such as a turbine jet, it is necessary to put two probes (emitter and detector) into the fluid flow.
This disrupts the flow away from what it is in use, without the probes. Of course, the size of the probes is kept small in the prior art, and the tests are performed with the probes in different positions to see how that effects the results.
According to a first aspect of the invention we provide an endoscopic optical fluid measurement probe assembly comprising an endoscope having a user end and a distal end, the distal end having a light emitter and a reflected light acquirer; and the endoscope being provided with transmission means to transmit information away from the distal end.
Thus, a single probe both emits light and detects light reflected by the fluid: both serves as emitter and data acquirer. This reduces the disruption in comparison with traditional two-probe PLSA systems. A single probe can also get into smaller spaces than can two probes and yield minimal intrusion.
Preferably the probe is adapted to emit laser light. Preferably the probe comprises a PIV probe or a LIF probe and the light emitter may comprise a light sheet producer adapted in use to produce a sheet of light. The probe may comprise one of: a PIV probe, a LIF probe, a stereoscopic 3-D PIV device, or any Planar Light Sheet Anemometer (PLSA). At present white light techniques do not provide as good results as laser light sheets, and laser light sheets is an important feature of many embodiments of the invention, but a non-laser light sheet may be envisaged.
Preferably the endoscope comprises a light-transmitting channel, such as a crystal, fibre optic, or fibre optic bundle, extending along its length and optically coupled to the light emitter. Preferably the optical channel, e.g. fibre optic, is adapted to be optically coupled to a laser, for example when a Continuous wave laser is providing the light source or when a low energy pulsed laser is used.
Preferably the light acquirer is optically coupled to image transmission optics adapted to transmit an optical image or signal detected by the light acquirer away from the distal end, and preferably to the user end of the probe. The transmission optics preferably comprise one or more of lenses, prisms, and mirrors, and preferably comprises at least one lens, and at least two elements of (i) mirrors elements, or (ii) prism elements. There may be at least two mirrors, and/or at least two prisms, or at least one of each.
The image transmission optics is arranged to transmit the acquired image to a camera or other imaging device provided outside of the confined space into which the distal end of the endoscopic probe has been inserted. Of course, if cameras became small enough to be introduced into confined spaces, and if they become robust enough to survive and operate in the environmental conditions to which they are exposed in use, the camera itself may be provided at the distal end of the endoscopic probe. For example a small CCD camera may be provided at the distal end of the probe, and image signals may be exported from the distal end (e.g. electrical signals or a wireless e.m. transmission). However, using optical components and transmitting the image itself away from the distal end to a camera remote from the distal end is preferred at present since the probe can be kept smaller/the mirrors/lenses/prisms can cope with more extreme environments.
There may be a first optical element with a reflective surface inclined at generally 45xc2x0 to a reflective surface of a second optical element. The light acquirer may have a pair of optical elements (e.g. mirrors or prisms) with reflective surfaces extending generally parallel to each other.
The light acquirer may have an optical element having a reflective surface, and the light emitter may have an optical element having a reflective surface, and the reflective surfaces may be inclined relative to each other so as to create a light output in a first plane that is substantially normal to the line of sight of the optical element of the light acquirer. It will be appreciated that the line of sight of the light acquirer is the line that is reflected up the endoscope, in use of the endoscope. The reflective surfaces of the light acquirer and light emitter may be inclined at an angle between 0xc2x0 to 90xc2x0 to each other, or at another angle that may be by 90xc2x0, or may not be 90xc2x0.
The endoscope may have an elongated body, preferably tubular, and preferably provided with an extension, arm, or foot at or towards its distal end. The foot may extend away form the body, possibly at about 90xc2x0 to the body.
The light emitter may include an offset arrangement such that the light emitted in use passes transversely across the elongate body, with the light acquirer preferably being provided in the body. Alternatively, the light emitted by the light emitter may be arranged to propagate generally parallel to the elongate direction of the body.
The light acquirer preferably has a movable, preferably angularly movable, reflector. The reflector may be provided on a carrier. The carrier is preferably angularly movable about a first axis. The reflector is in some embodiments preferably angularly movable relative to a carrier about a second, different, axis.
For stereoscopic measurements an embodiment of the endoscopic probe has a first and a second arm or foot, typically at or towards its distal end, and each arm/foot has an image acquirer. In the arrangement of a preferred embodiment the image acquirer of the first arm acquires an image from one side of the sheet of light and the image acquirer of the second arm acquires an image from the other side of the sheet of light. This allows the out-of-plane velocity of particles to be established (velocity transverse (e.g. perpendicular) to the plane of the sheet of light).
Although any light source could be used, a laser is usually the most preferred source to produce a narrow and intense light sheet. Laser beams constitute well-collimated sources of intense light and they can very easily be transferred into a sheet using cylindrical lenses or scanning mirrors. Continuous or pulsed laser may be used depending on the technique to be applied. Argon lasers are good choices of continuos light and Ruby or ND-Yag lasers are chosen when a pulsed source is needed. The latter nowadays replaces more and more Ruby lasers since it allows easier focusing of a camera.
The energy available is best used by creating a very narrow light sheet. This may be accomplished by adding spherical positive lenses to the optical system to reduce the divergence of the laser beam.
The continuous wave or low energy pulsed light sheet may be transmitted into a confined space (e.g. a cavity) using fibre optics, alternatively (for example where the light source is a high energy pulsed laser) the beam can simply be directed into the cavity/combined space via mirrors, lenses and prisms.
The, or each, light acquirer may have a line of sight that is substantially perpendicular to the plane in which, in use, a sheet of light will be created by the light sheet producer. This gives the strongest reflected signals (reflected off particles in the fluid being measured), and may make the mathematics of the analysis software more straightforward.
The transmission optics may direct an image, or optical signals, into a camera, a sensor, or an array of sensors, which may comprise part of the probe assembly, or the probe assembly may not include them and instead be adapted to be coupled to them. The camera may be a photographic film camera. The camera may alternatively be a charge coupled device camera. The camera may communicate with a time delay and sequence generator to allow it to record images at predetermined intervals. The assembly preferably further includes means to analyse the recorded images. The means of analysis of the recorded image(s) is preferably a computer and appropriate software, to which signals from the camera (or sensor or sensor array) are fed.
The present invention according to another aspect comprises a method of determining a parameter associated with a fluid in a restricted space and/or a hostile environment, comprising using an endoscopic optical probe having an elongated length and provided with an optical light emitting element towards its distal end and provided with an optical light acquiring element towards its distal end, the probe being adapted to emit light via the light emitting element and collect light, via the light acquiring element, that has been emitted by the emitting element and interacted with the fluid; and transferring acquired light, or other signals, away from the distal end, for example along the endoscopic probe to a remote sensor, remote from the distal end of the endoscopic probe; and processing signals produced by the remote sensor to evaluate a parameter of the fluid.
Preferably the processing comprises applying one of the following techniques to the signals: PIV; LIF, PLIF, MLIF, stereoscopic PIV, or any PLSA technique.
Preferably, the method comprises determining at least one of the following in an environment that is hostile or conventionally inaccessible; such as an engine bearing (preferably an aircraft engine bearing): fluid flow; particle analysis; and temperature.
The present invention further provides a method of determining a parameter associated with a fluid (such as fluid velocity, temperature, or particle size and concentration) comprising the steps of:-
producing a first light sheet;
transmitting the first light sheet into the fluid using an endoscope;
recording the image illuminated by the first light sheet using an image acquirer provided on the same endoscope.
Preferably the method further comprises:
producing a second light sheet;
transmitting the second light sheet into the fluid using the endoscope;
recording the image illuminated by the second light sheet;
analysing the images illuminated by the first and second light sheet and determining the required parameter.
The means to produce the light sheet is preferably a laser light source with optics. The preferred light source is a laser. The light sheet may be transmitted into the fluid, or confined space or cavity, using fibre optics or alternatively where the light source is a laser fibre optics may not be necessary: the laser beam can simply be transmitted into the cavity.
The fluid may have particles in it, for example tracer particles, which may be deliberately introduced into the fluid. The shutter of the camera may be opened for a sufficient time, synchronised with pulses of the laser, to produce snapshots separated in time. Particles may therefore be present in two images, showing the position of the particle each time of firing the laser.
It may be important that light is intense enough for the pulse duration to be as short as possible to avoid any blurring of the image of the tracer particles, and that the film used in the camera (if it has a film) is sufficiently sensitive for the wavelength of the laser. In most cases, a pulsed laser is used, but for relatively low speed flows (less than 10 m/s) it is possible to use a continuous laser such as an Argon laser together with a mechanical or opto-electronical shutter to generate the required pulses.
In most cases, the particles should be small enough to follow the flow of fluid in the cavity or other confined space (e.g. tube). They will therefore scatter a small amount of light. Furthermore, this light may conveniently be collected at 90 degrees to the incoming illuminating light.
The photographic camera can be replaced with a CCD (charge coupled device) camera. The advantage of CCD cameras is the possibility of on-line processing of the images of the flow. The disadvantage is the low resolution of the camera presently available, which limits the size of the interrogation area. However, resolution may well improve in future CCD cameras.
High-speed image capture and data transfer to the computer are accomplished by the use of hardware processors or by software. The latter lends itself better for flexibility, accuracy, new developments and other novel analysis schemes preferred by users. Real time, or nearly real time, analysis may be possible.
According to an other aspect the invention comprises performing LSV, PIV, LIF, PLSA, stereoscopic 3-D PIV, or other optical fluid flow analytic techniques using the same endoscope to emit light and to detect reflected light.
According to another aspect the invention comprises the use of an endoscopic probe assembly in accordance with the first aspect of the invention in the performance of a fluid flow analytical technique.
According to another aspect of the invention, we provide a fluid analyser system comprising a probe in accordance with the first aspect of the invention coupled to a laser or other light source, and coupled to at least one camera, or two cameras, or to another detector(s).
Preferably the system comprises a control device, such as a computer or microprocessor which in use controls the operation of the laser and receives signals from the detector. Preferably the control device is also adapted, in use, to control the movement of the endoscope.
It will also be appreciated that an endoscopic probe having optical light emitting and light capturing elements at its distal end and light transmission means along its elongate length allows the provision of the sensitive camera/detector, and sensitive and bulky laser and power supply and control, remote from the distal, inspection, end which can therefore be placed in hostile environments. So long as the light emitting and capturing elements are capable of withstanding the conditions, and the body of the endoscope itself can withstand the conditions, the distal end can experience conditions that would destroy the camera/detector.
Since making the invention we have become aware from a UK Patent Office Search of GB 1 545 699 which discloses a probe, but not a light sheet and is not PIV, and does not handle images; U.S. Pat. No. 5,202,558 which does not relate to light sheets and is not a pulsed system, and is not PIV; WO 93/19376 which is not a miniature probe, not PIV, has no light sheet emitted, and is not imaging in a 2-D sheet; WO 95/33999 which deals with beams not sheets, is not a single probe; and does not image particles; EP 0 394 602 which is not a miniature probe and does not have the same single probe transmit and receive light and detected images; GB 2 213 018 which has no light sheet, does not image, and is not PIV; and GB 2 339 107 which has two beams, has no sheet of light, does not image particles, and is not PIV.